It is proposed to investigate (a) the association of leukemia with agricultural occupation, particularly occupational contact with poultry and (b) the occurrence of human infection with Marek's Disease Virus (MDV), a DNA herpesvirus widespread among poultry. (a) All persons dying with a diagnosis of leukemia, lymphoma, and pancreatic carcinoma (as a methodologic control), together with matched controls, will be identified from the death certificate registries of Illinois, Indiana and Wisconsin. Life history of residence and occupation will be obtained by mailed questionnaires and telephone inquiry from survivors of all decedents. Risks of developing study malignancies among farmers and their families relative to non-farm individuals, and of poultrymen relative to other farmers and non-farmers, will be determined. (b) MDV infection will be investigated by serological and virologial methods. After establishing the specificity of the neutralizaton and membrane fluorescence tests, using specific antisera prepared in rabbits and cell- free infectious MDV, HSV, CMV and V/Z virus, the most sensitive and specific assay method will be used to test for presence of antibodies in humans with and without contact with poultry. To determine if masked MDV is present in human leukemic cells, such cells from poultrymen and veterinarians with leukemia will be cocultivated with chicken or duck kidney cells in vitro. The resslting hybrid cells will then be examined for infectious virus, for MDV antigen, and for virus particles visualized by electron microscopy.